The present invention relates to an electric discharge machining apparatus for forming holes of various shape in a workpiece by causing electric discharge between a tool electrode and the workpiece and moving the tool electrode towards the workpiece.
Electric discharge machines are widely used to accurately machine a solid conductive workpiece into a mold or a die. The workpiece is fixed to a table arranged in a work tank, and a tool electrode of copper or graphite is attached to a vertically movable quill or ram using a tool holder. The work tank is filled with dielectric fluid such as kerosene, and the tool electrode is positioned extremely close to the workpiece. The space between the workpiece and the tool electrode is known as the machining gap, and the size of this gap typically ranges from a few xcexcm to a few tens of xcexcm. If a power pulse is applied across the workpiece and the tool electrode, during the pulse ON time, the insulation characteristics of the dielectric fluid in the gap break down and electric discharges occur. Microscopic amounts of the workpiece material are evaporated or become molten due to heat of this electric discharge, and flow into the dielectric fluid. Upon completion of the pulse ON time, i.e., during the power pulse OFF time, the insulation characteristics of the dielectric fluid in the gap are restored. As a result of the electric discharges produced during the pulse ON time, microscopic crater shaped holes remain in the surface of the workpiece. An electric discharge machining apparatus normally controls the pulse ON time and OFF time between 1 xcexcsec and a few tens of msecs, whereby power pulses are repeatedly applied to the gap. The electric discharge machine causes the tool electrode to move towards the workpiece and along the Z axis in order to maintain the gap at a relatively constant size. Since it is possible to remove microscopic amounts of material from the workpiece without the tool electrode coming into contact with the workpiece, a cavity having good surface roughness and a shape complimentary to that of the tool electrode may be accurately formed in the workpiece. This type of electric discharge machine, known as a sinker EDM, is different from a wire EDM which uses a moving wire electrode.
During electric discharge machining, a flushing operation to produce a flow of dielectric fluid through the gap in order to wash fragments removed from the workpiece away from the gap is important. The flushing operation serves to prevent undesirable secondary discharges from occurring between the tool electrode and the fragments removed from the workpiece, and restores reliable insulation during the power pulse OFF time. A skilled operator may form holes for sucking contaminated dielectric out of the gap and supplying fresh dielectric fluid into the gap at appropriate positions in the tool electrode and the workpiece. When formation of the holes is restricted, such as because of the size or shape of the tool electrode, the operator may arrange an injecting device at an appropriate position to inject dielectric fluid towards the gap. Flushing is crucial in order to carry out good machining faster and more accurately; but creating a uniform flow throughout the entire gap requires skill. A technique known as a xe2x80x9cjumpxe2x80x9d operation, where the tool electrode is caused to periodically rise up rapidly and fall down rapidly along the Z axis may be used to almost totally expel contaminated dielectric fluid inside the gap from the cavity inside the workpiece. During the jump operation, the tool electrode conventionally moves at a rate of a few hundred mm/min. If the reciprocating distance of the tool electrode is sufficiently large, more fresh fluid flows to the gap and more contaminated fluid is expelled from the gap. The tool electrode is preferably made to rise up by at least a depth of a hole being machined in the workpiece. However, since material is not removed from the workpiece during the jump operation, carrying out the jump operation too often will cause the stock removal rate to be lowered.
An object of the present invention is to provide an electric discharge machining apparatus that can effectively flush fragments removed from a workpiece away from a gap without lowering the stock removal rate.
Another object of the present invention is to provide an electric discharge machining apparatus that can effectively flush fragments removed from a workpiece away from a gap without the need for a high level of skill by the machine operator.
Additional objects, advantages and novel features of the invention will be set forth in the description that follows, and will become apparent to those skilled in the art upon reading this description or practicing the invention. The objects and advantages of the invention may be realized and attained by practicing the invention as recited in the appended claims.
In order to achieve the above and other objects, an electric discharge machine according to the present invention, for machining a workpiece by causing a tool electrode to move towards the workpiece in a vertical direction while causing electric discharge between the workpiece and the tool electrode, may comprise; a quill movable in the vertical direction, an electrode mounting device attached to a lower end of the quill coaxially with the quill for mounting the tool electrode, at least one set of linear motor movers attached to the quill and arranged symmetrically around the central axis of the quill, and a set of linear motor stators respectively facing the set of movers.
Preferably, the set of movers includes magnetic plates attached to the quill and a row of permanent magnets arranged on the magnetic plates, and the set of stators includes a yoke and a coil wound around the yoke.
More preferably, the quill has a hole in its center extending in the vertical direction, and an air cylinder for obtaining load equilibrium of the quill is arranged in this hole.
In another aspect of the present invention, an electric discharge machine, for machining a workpiece by causing a tool electrode to move towards the workpiece in a vertical direction while causing electric discharge between the workpiece and the tool electrode may comprise; a quill movable in the vertical direction having a density of less than about 4 g/cm3, an electrode mounting device for fixing the tool electrode attached to a lower end of the quill, a linear motor mover attached to the quill, and a linear motor stator facing the linear motor mover.